Type I Glanzmann thrombasthenia caused by an apparently silent beta3 mutation that results in aberrant splicing and reduced beta3 mRNA

Abstract

We report a novel genetic defect in a patient with type I Glanzmannthrombasthenia. Flow cytometry analysis revealed undetectablelevels of platelet glycoproteins αIIb and β3,although residualamounts of both proteins were detectable in immunoblottinganalysis. Sequence analysis of reversely transcribedplatelet β3 mRNA showed a 100-base pair deletion in the3’-boundary of exon 11, that results in a frame shift and appearanceof a premature STOP codon.Analysis of the correspondinggenomicDNA fragment revealed the presence of a homozygousC1815T transition in exon 11.The mutation does not changethe amino acid residue but it creates an ectopic consensus splicedonor site that is used preferentially, causing splicing out of part of exon 11.The parents of the proband, heterozygous for thismutation,were asymptomatic and had reduced platelet contentof αIIbβ 3. PCR-based relative quantification of β3 mRNA failedto detect the mutant transcript in the parents and showed amarked reduction in the patient. The results suggest that thethrombasthenic phenotype is, mainly, the result of the reducedavailability of ß 3-mRNA,most probably due to activation of thenonsense-mediated mRNA decay mechanism.They also showthe convenience of analyzing both genomic DNA and mRNA, inorder to ascertain the functional consequences of single nucleotidesubstitutions